Ram Problem

Boot Computer and when the Startup screen appears,press the key to Bios setup,click on Advanced,then check the memory setting,and change the setting to maximum amount of memory installed

Check the motherboard manual and see if the specified amount is less than what is noted that can be actually installed on the board.

The last option would be to update the bios,by using the manufacturers Flash utility

What OS are you running?

There are known problems with W9X and over 511MB of RAM.

I limit my 9X systems to 384MB (256 +128) to avoid the problem.

It seams to be system specific as to what the maximum amount of functional RAM is.

Check out Microsoft Knowledge Base articles;
Q184447
Q253912

For more details.

As a side note, W98SE seams to run the fastest with 196MB on some motherboards.
Many TR peers report that W98 does not "effectively" handle memory above 128MB.

How to get past this issue? Switch to XP.

Are you using Win9x, i think there is a bug, in Win9x, using greater than 512Mega Byte of memory, can cause a file corruption, crash..., system halting,,,and a lot....

here :
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q184447

there is a resolution and hope you try that...
If does nt work.....

I think you need to upgrade your OS to a newer version.

Hope this can..

SYRUPHINE

You don't say what kind of system you are running but if it was one of the "lot" buying manufacturers your actual MB specs may be somewhat different then what your manual and even bios state. Packard Bell was famous for this, Gateway does it sometimes too. Was this the original configuration or are you adding extra memory? Does this board have jumpers to enable memory slots?
the last thing to check would be the bios. find out what version you have at startup the check the vendor web site for the latest version and/or any issues that have come up with the version you are currently using.